For a steering device, use ids made of such a steering device as shown in FIG. 7 in order to transfer rotation of a steering wheel to a steering gear. As shown in FIG. 7, the steering device is provided with a steering shaft 12 in which a steering wheel can be mounted, the steering shaft 12 being on the rear side of a vehicle body (on the right-hand side in the FIG. 7), a steering column 13 with the steering shaft 12 inserted therethrough, and a steering gear coupled to the steering column 13 via a rack and pinion mechanism (not shown), the steering gear being on the front side of the vehicle body (on the left-hand side in the FIG. 7).
With the steering shaft 12, a female steering shaft (female shaft) 12A is serration (or spline)-fitted onto a male steering shaft (male shaft) 12B in such a way as to cause a rotation torque to be transferable, and these members to be relatively movable in the axial direction of the steering shaft 12. Accordingly, the female steering shaft 12A and the male steering shaft 12B causes a spline-fitted joint to make a relative movement at the time of a collision, thereby enabling the whole length of the steering shaft 12 to be contracted.
Further, the steering column 13 cylindrical in shape, with the steering shaft 12 inserted therethrough, has a so-called collapsible structure. More specifically, an outer column 13A and an inner column 13B are combined with each other in such a way as to be telescopically movable, and if an impact in the axial direction is applied thereto at the time of a collision, the steering column 13 undergoes contraction in total length while absorbing energy of the impact.
The outer column 13A is supported by a part of a body 18, such as the underside of a dashboard, and so forth, through the intermediary of an upper support bracket 14. Further, a stopper (not shown) is provided between the upper support bracket 14, and the body 18 such that the upper support bracket 14 is dislodged from the stopper to be movable toward the front side of the body when an impact in a direction toward the front side of the body is applied to the upper support bracket 14. Further, an end of the inner column 13B, on a side thereof, adjacent to the front of the body, is also supported by a part of the body 18 through the intermediary of a lower support bracket 19.
An end of the female steering shaft 12A, on a side thereof, adjacent to the front of the body, is coupled to the rear end of an intermediate shaft 16 via a universal coupling 15. Further, an input axis of a steering gear (not shown) is coupled to the front end of the intermediate shaft 16 via another universal coupling 17. With the intermediate shaft 16, an end of a male intermediate shaft (male shaft) 16B, on the rear side of the body, is serration (or spline)-fitted into an end of a female intermediate shaft (female shaft) 16A, on the front side of the body, thereby permitting these members to be fitted with each other in such a way as to enable a rotation torque to be transferable, and to be relatively movable in the axial direction.
The rotation of a steering wheel 11 is transferred to the steering gear via the male steering shaft 12B, the female steering shaft 12A, the universal coupling 15, the female intermediate shaft 16A, the male intermediate shaft 16B, and the universal coupling 17, thereby causing wheels (not shown) of a vehicle to be steered.
The steering device described as above need to have a structure constructed such that at a time when the male shaft is serration (or spline)-fitted into the female male shaft upon occurrence of a vehicle collision to undergo relative contraction, resistance to collapsing in the middle of the contraction is increased to thereby increase an absorption amount of impact energy, enabling the impact energy to be sufficiently absorbed even at a short collapse-stroke. In Patent Document 1, there is disclosed such an impact-absorption type steering shaft as shown in FIG. 8.
FIG. 8 (a) is a sectional view showing principal parts of the male shaft 12B, and the female shaft 12A, shown in FIG. 7, respectively, and FIG. 8 (b) is an enlarged perspective view of a part R shown in 8 (a). As shown in FIG. 8 (a), a male serration 20 formed on the outer periphery of the left-end of the male shaft 12B is fitted into a female serration 30 formed on the inner periphery of the right-end of the female shaft 12A to be brought into serration-engagement with the latter.
A protrusion 23 is formed in a recess 22 between tooth 21 of the male serration 20, positioned at a part of the male serration 20, in the axial direction thereof, located at a spot in the circumferential direction thereof. The position of the protrusion 23, in the axial direction of the male serration 20, is a position where the protrusion 23 will not come into engagement with the female serration 30 at the time of a normal driving operation, allowing the protrusion 23 to come into engagement with the female serration 30 when the male shaft and the female male shaft undergo relative contraction at the time of a vehicle collision.
If the female male shaft 12A, and the male shaft 12B undergo relative contraction at the time of a vehicle collision, the protrusion 23 of the male serration 20 is brought into engagement with a serrated edge of the female serration 30 to thereby cause the serrated edge of the female serration 30 to undergo plastic deformation. As a result, the resistance to collapsing will increase, thereby causing an absorption amount of impact energy to be increased.
With the impact-absorption type steering shaft described in Japanese Utility Model Registration No. 2607069, an absorption amount of impact energy can be increased by increasing the resistance to collapsing in the middle of contraction. However, since the protrusion 23 is provided only at a specified spot in the circumferential direction of the male serration 20, it has been difficult to set the absorption amount of impact energy to a predetermined magnitude because the absorption amount of impact energy varies depending on in which phase in the circumferential direction of the male serration 20 the protrusion 23 will be present.
Further, as shown in Japanese Unexamined Patent Application Publication No. Hei8 (1996)-91230, there is available an impact-absorption type steering shaft having a configuration such that a female steering shaft 12A, and a male steering shaft 12B undergo plastic deformation into the shape of an ellipse in cross-section by pressing down the outer peripheral surface of the female steering shaft 12A inward in the radial direction thereof to thereby absorb impact energy. However, if a structure of Japanese Unexamined Patent Application Publication No. Hei8 (1996)-91230 is applied to the structure of Japanese Utility Model Registration No. 2607069, this will render it difficult to align the position of the protrusion 23, in the circumferential direction of the male serration 20, with the phase of the ellipse formed due to the plastic deformation, so that it has been difficult to set the absorption amount of impact energy to a predetermined magnitude.    Japanese Utility Model Registration No. 2607069    Japanese Unexamined Patent Application Publication No. Hei8 (1996)-91230